So here's why... (I think... it's close, but any electricians out there feel free to interject...)

Each solar panel is made up of a multitude of cells. Each cell produces about 0.5V, so a 12V panel normally has about 36 cells wired in series - to give a charging potential of 17V. Amps are generated according to the amount of sunlight - but following Kirchoff's laws for a series wired circuit, the current in each cell is the same. If you short circuit one cell, you effectively short circuit the entire cell. Output drops to zero. If you use the same shade area but apply it at the intersection of 4 cells, partially shading them all, the reduction in output will be less marked since only 25% of the area of each was shaded...

For a 12V system, with 2 panels, wire them up in parallel so that if one panel is shaded, output from the other will remain good. If wire in series, than shading of just one cell of the two panels will kill all output.

If you were to wire up 2 in series, you'd end up with an output voltage of 34V - wouldn't that harm the batteries? Just how smart are the solar controllers?

Bill Balme

Yes, I was having a bit of a senior moment yesterday-in order for the voltage to remain at 12v they have to be wired in parallel. The impact of shading was quite interesting though. Cheers.

Here's the link to the youtube video. It's certainly an interesting video...

My first instinct is that wiring in series would be a real problem because the voltage would be too high. Unfortunately this couple did not seem to care at all about voltage...

The effect of shading one square of either of the two panels is amazing - dropping current by 50% - I would not have expected this and would love to hear a reason.

Bill Balme

Over on the OCC Facebook page Anne Lloyd has posted a link to a YouTube video showing an experiment with solar panels wired in parallel and then in series, and the effect of shading. It seems to demonstrat a significant performance advantage from wiring in parallel. It would be interesting to get some expert (or even not so expert) views on here as well as on Facebook.